Frankenstein's Prop Pitch Motor

When we last visited my malfunctioning prop pitch motor, a variety of problems were evident. After that article was published I proceeded with a complete disassembly of the gearbox. That uncovered more problems; poor low-temperature performance was the least of them. 

Several bearings were in poor shape. Among those are the 6 bearings supporting the 3 low-speed planetary gears. According to K7NV, perhaps the foremost expert on prop pitch motors, they have no modern equivalent. I was able to confirm that with extensive catalogue searches. Note that I'll be citing the K7NV web site numerous times in this article. If you have an interest in prop pitch motors you'll enjoy browsing his web pages.

I can try to repair the bearings or I can fabricate shims to accommodate the closest alternative. With the oil and grease removed in a solvent bath there are two of the group that exhibit damage to the races and/or balls serious enough to warrant repair or replacement.

Another difficulty is the sleeve on the motor end of the gearbox axle. It has splines that match the motor shaft splines and the sleeve cannot be removed with any jig I could cobble together in the workshop. I'll have to take it to a professional.

None of these are insurmountable problems, but it will take time. Summer is running away from me and I need the 15 and 20 meter stacks to be fully operational before the fall contest season. There are many other jobs to be done and I can't spend all my time on one motor. 

In the midst of these woes the elderly father of a friend of a friend was downsizing and had a small prop pitch motor for sale. After speaking on the phone and reviewing pictures of it I went ahead and bought it, sight unseen. The price was attractive, especially since it came with a DC power supply and rudimentary direction indicator. A friend picked it up and saved me a long drive.

When it arrived I connected it to my existing controller and got a big surprise. The motor turned at 4 or 5 rpm. That's much faster than the more typical rotation speed of 0.6 to 0.75 rpm. I called the seller and he, too, was surprised.

He had never opened the gearbox and had no idea that it was performing in any way that was unusual. The motor has a long pedigree of many owners, lost in the mists of time. Like those earlier owners, he controlled the speed with a Variac or similar voltage rheostat; that is; with a voltage below the nominal 24 VDC. Not everyone is a prop pitch motor expert -- it works to their expectations and that's that!

I refused his offer to refund my money. The spare parts and electronics are worth the price. However there was still a mystery to be solved. I had a vague recollection that some early owners of prop pitch motors modified them to run at a higher speed, or lower voltage. I had never come across one before and I was curious how it was done.

It turns out that the way it is done is horrifying. A friend dug through his extensive prop pitch motor library and found an article from a 1949 issue of CQ magazine that described the mod. It's a non-reversible mod that destroys a critical component of the gearbox. 

With trepidation I opened the gearbox. I could have been more careful about it because a quantity of machine oil poured out and splashed over the workshop bench and floor. Almost everyone who converts these motors for rotator service drains the oil and does not replace it. The oil is removed to prevent fouling the motor when it is mounted upside down. The oil seals are not perfect and the original bearings are open and need to splash through the oil reservoir. 

The usual procedure is to replace the original bearings with modern sealed bearings and to grease the gears. That wasn't done for this motor. However, the oil seals seemed to be working well.

After cleaning the mess I discovered that the motor was indeed modified per that 1949 article. In the picture I am pointing at the cut edge of the large bell gear. The bell gear of the motor being repaired is shown for comparison. 

The smaller diameter gear of the low-speed planetary gears have nothing to mesh with. Steel plugs (one is shown) directly couple the oil channels of the bell gear and the carrier for the low-speed planetary gears. One stage of speed reduction is thereby eliminated.

It is an atrocity to do this to a perfectly good prop pitch motor. Or is it? A digression to review the historical context is enlightening. 

After WW II, these motors were often easily and cheaply acquired on the US military surplus market. Indeed, many ham shacks of the late 1940s and 1950s were filled with war surplus transmitters, receivers, cables and other odds and ends. Few hams had towers and yagis, and large rotatable HF yagis were very rare indeed. It was still the early days of broadcast television and the mass market for rotatable TV antennas and rotators. Many hams made their own rotators.

A 24 VDC power supply was easily built, and everyone knew someone who could do a little machining and welding to adapt the motor to a tower plate and mast. But the motor turns slowly and it makes quite a lot of noise for a suburban neighbourhood. Bypassing one stage of reduction in the gearbox speeds up rotation too much, so the motor voltage is reduced to compensate. The motor is much quieter when run at only 1000 to 2000 rpm. Problem solved. That is, until a ham like me comes along decades later.

So, the modification is not an atrocity, just a disappointment. I considered options. The motor mounts and housings are of a different design and it was no simple matter to substitute components from one motor housing to the other. Over the ~25 years these motors were produced there were a variety of engineering changes, not all of which were backward compatible. The mast drive system on the new motor is well done (see pic at the bottom of the article) but it, too, is not easily retrofit to my mast and I am not going to take down the 15 and 20 meter yagis and mast to do it.

I pulled the planetary drive assembly from the gearbox to compare it with the malfunctioning unit. They are identical, right down to the part numbers stamped or printed on the components. All the bearing and gears were working properly.

I decided to mix parts from the two dead or half-dead prop pitch motors. Hence the reference to Frankenstein in the article title. 

The critical step was to drop the newly acquired planetary drive assembly into the old housing. That sounds straight-forward, except that I ran into a few complications.

The first thing I did was a cold temperature test of the assembly. As I did to diagnose the problems with the one I pulled down from the tower, I placed it in the freezer overnight. Not all oils and greases have a temperature range to match our climate.

The next morning I checked all the bearings and they spun freely. The oil didn't noticably thicken. Condensation moisture gradually evaporated and did not appear to foul the oil residue. This is important since despite the best waterproofing moisture will condense inside the housing at night when the relative humidity is high. Condensation and freezing fouled a couple of the open and improperly greased bearings in the old planetary drive assembly, which contributed to its poor low-temperature performance

Without the oil bath to keep the bearings and gears lubricated, grease would have to be applied. I was not prepared to fully disassemble the planetary drives to replace the bearings with sealed units. It would take too long and there was a risk of damaging something.

I may pull the motor down again next year to inspect the durability of the lubricants and bearings. At this point I simply need it to survive the coming winter and contest season.

I bought suitable wide-temperature range oil and grease, oiled the bearings and packed grease where I could. Gear cogs were heavily coated with a water-resistant grease as recommended by K7NV.

In the picture the oiled and greased planetary gear assembly is pressed into the motor side of the old housing, along with the ring gear that is sandwiched between the halves of the housing. The only thing left is to press the large bell gear onto the assembly and the gears of the low-speed planetary drive. That did not go well.

There is a trick to mounting the bell gear that is due to the asymmetry between the gear cogs that engage the ring gear and those that engage the bell gear. I assumed they were aligned since the assembly showed no obvious sign of having been taking apart in the past. No matter what I did the bell gear would not fit.

Checking the alignment was a messy job since the gears were heavily greased. There are alignment marks on the 3 low-speed planetary gears that must all be oriented outward. I posed one of these gears of the disassembled planetary system to illustrate the procedure.

I removed the ring gear and rotated the planetary drive while wiping grease off the upper surface, searching for the alignment marks. Two of the gears were aligned but not the third. It had been disassembled in the past and not correctly reassembled. You can do that with the modified gearbox because the ring gear will engage the planetary gears no matter how they're oriented.

With a long sigh I removed the nut from the axle and pried off the top bearing and low-speed planetary drive. With the cogs disengaged from the gear on the bell gear for the high-speed planetary drive, the gears were aligned and then carefully pressed back onto the axle and spur gear. Despite being covered in grease up to my wrists I grinned when the bell gear easily dropped onto the planetary drive gears. 

I repacked the grease and proceeded to finish the job. Mostly this consisted of cleaning the remaining half of the housing and renewing the seals. Waterproofing the coupling to the mast is a separate task that I've already begun. I temporarily used a small number of bolts to hold the housing together and secure the motor to the housing. There were two tests to be done before the rebuild could be considered complete.

It was more convenient to take the motor to the controller than the opposite. Well, sporadic E season isn't quite over and it's easy to monitor or operate digital modes on 6 meters while testing the prop pitch motor. I gave it a lengthy spin in both directions and it behaved as it should. Rotation speed is the same as before, which is about 100 seconds to turn 360°. The motor on the other tower does the same in 80 seconds. I have an idea why it's slower but that will have to wait for another time, perhaps in 2023.

I removed the motor and freezer tested the reassembled gearbox for low-temperature performance. That went well so I remounted the motor and installed all the bolts holding the components together.

Assuming no new obstacles intervene, the motor should be back in service by mid-August. I am making a few changes to the mast coupling system to better centre the mast coupling and to keep water out of the motor. Although the drive system appears to be well sealed there's clear evidence that some water is leaking into the crown gear and from there into the gearbox. That's another good reason to use sealed bearings throughout.

I cleaned the waterborne rust that coated the inside of the hub. It was washed down from the unpainted coupling pipe. The high quality steel comprising the prop pitch motor housing and parts is almost immune from rusting.

I'll resume work on the disassembled gearbox when I have time this winter. I plan to rebuild it with sealed bearings and, hopefully, repair the damaged bearings on the low-speed planetary drive. That will give me more prop pitch motor service options for next year and beyond.

One final point worth mentioning is that the recently acquired motor has an adapter plate. This is ideal for keeping water out when mounted upside down for rotator service. Unfortunately I had to set it aside since the mast coupling system I built isn't compatible with it. It seems a shame to waste so I may try a retrofit in future.

I've learned a lot about prop pitch motors while doing this repair job. I am no longer shy about cracking them open and getting my hands dirty, literally! They are truly impressive devices.